Telecommunication networks were originally developed to provide voice communications between subscriber terminals. With the experience of such networks from the traditional telephone or POTS service into the provision of a variety of digital series, there is now a need to provide improved subscriber access to the network. Traditionally subscribers are coupled to the network with a twisted pair wire loop commonly referred to us the subscriber loop. The costs of replacing these loops with more efficient or higher bandwidth connections, e.g. a coaxial connection, is prohibitive and various techniques have therefore been proposed for providing effective digital across over the subscriber loop. A recently introduced transmission technique is the orthogonal frequency division multiplex (OFDM) protocol. This technique comprises a multi-carrier modulation scheme which achieves a very low co-channel interference over dispersive channels by the use of a cyclic prefix on each data symbol. In contrast to conventional frequency division multiplex (FDM) techniques, no complex time division equalisation is required. A description of this technique is given by J Bingham in IEEE Communications Magazine 28(4) pp 5-14 --April 1990 and by A Peled and A Ruiz in `International Conference on Acoustics, Speech and Signal Processing` April 1980, Denver, pp 964 to 967. Further descriptions of OFDM techniques are given in specification No EP-A2-0656705 and specification No U.S. Pat. No. 4,884,139.
Orthogonal frequency division multiplex is ideally suited to digital transmission applications, and has been implemented using standard fast Fourier transform (FFT) techniques. However, the application of this technique to telecommunications subscriber loop technology, where the transmission path comprises a twisted copper pair, has been restricted by its susceptibility to high level tone interference. In such circumstances, the rejection offered by conventional FFT processing has been found insufficient to prevent significant data corruption.
The object of the invention is to minimise or to overcome this disadvantage.